Porous plastic films can be grouped into two different types: (1) macroporous films which are both vapor and liquid permeable and (2) microporous films which are vapor permeable but impermeable to liquids.
Various processes have been disclosed for the manufacture of porous films of both types, all of them involving the formation of pores or perforations extending through the film from one film surface to the other.
One group of such processes entail the formation of an unfilled thermoplastic resin film and then perforating or piercing the film during its passage through the nip of two rollers. For instance, in the process of U.S. Pat. No. 3,302,501, the piercing objects are pins or needle-like projections extending from the surface of one or both rollers. Other processes utilize pairs of rollers, one of these having a smooth, often heated surface, and the other having either projections in its surface as in U.S. Pat. No. 3,073,304 or an abrasive surface coating as in U.S. Pat. No. 3,718,059. The latter processes have not been entirely successful because of the manufacturing cost of the rollers having surface projections and also because of operational problems causing undesired large variations in the film pore diameters. In addition, the improvements in the moisture vapor transmission rates have been less than desired using the smooth pressure roller techniques.
A second group of such perforation processes employs differential pressure, e.g., vacuum, between the surfaces of a heated film to be drawn into the openings of the screen and thereby become perforated. Such processes are disclosed in U.S. Pat. Nos. 4,351,784 and 4,456,570. The resulting perforated films are macroporous and are liquid permeable.
A third group of processes accomplish the perforation by first forming a film of an incompatible polymer matrix, either polymer-to-filler as in U.S. Pat. No. 4,472,328 and British Application No. 2,151,538, or incompatible polymer-to-polymer as in Japanese Patent No. 73-28789, and then stretching the film in one or more directions causing the formation of microtears in the film.
There is also a process disclosed in U.S. Pat. No. 4,364,985 for preparing a porous microfilled thermoplastic film, wherein the inorganic filler particles are required to have an effective diameter larger than the thickness of the film. Small spaces are formed around the particles and when the filled film is passed over a set of serially positioned abrading rollers for the purpose of rubbing off the surfaces of the film that overlie the outermost portions of the filler particles, there is formed a multitude of fine pores extending from one film surface to the other. This technology has severe extrusion problems due to particle size requirement and hence weak properties.
U.S. Pat. No. 3,880,966 discloses a process for the manufacture of microporous films which involves the corona treatment of a nonporous, elastic starting film followed by a series of necessary steps comprised of cold stretching, hot stretching and heat setting to render to the corona treated film a microporous structure. The physical properties of these films, especially then the strength and hydrostatic pressure resistance, are not sufficiently high to utilize the films for many applications.
Also, corona treatments have been disclosed as useful in further improving the wettability and thus the liquid permeability of perforated macroporous films, such as the vacuum formed perforated films previously mentioned in U.S. Pat. Nos. 4,351,784 and 4,456,570.
It is therefore, an object of the present invention to provide a novel process for the production of a porous film which is both moisture and liquid permeable as measured by moisture vapor transmission rate and hydrostatic pressure resistance.